In addition to screens and settling tanks which, in a first step, free wastewaters from coarse components, conventional sewage treatment plants also contain an activation tank and a secondary clarification tank. In the activation tank, microorganisms are used to decompose fecal matter or other organic substances. The microorganisms used are separated off again by sedimentation from the wastewater subsequently to the activation tank, in the secondary clarification tank and in part recirculated to the activation tank. Complete separation of the microorganisms is not possible by a sedimentation process, and therefore microorganisms which are harmful to health can sometimes pass into the environment together with the wastewater. European union directives for keeping water bodies clean, which guidelines have already been substantially converted to national legislation, however, prescribe on a Europe-wide basis that only biologically clarified wastewater which is essentially free from microorganisms may be allowed to drain off into the environment. The microorganisms are generally separated off by fine filters which reliably separate off microorganisms.
In Germany, primarily in rural areas, in particular in eastern Germany, predominantly for technical or economic reasons, currently an estimated 5.3 million people are not connected, to the municipal wastewater grid. Those affected, as a consequence of the EU directives, had to be provided with facilities by Dec. 31, 2005, either by connection to the municipal grid or by the operation of a small sewage treatment plant. This situation created a high demand for decentralized wastewater treatment plants which is maintained to date.
Small sewage treatment plants are relatively widely distributed, which sewage treatment plants consist of a round concrete container which again is subdivided into three chambers (preliminary clarification/activation tank/secondary clarification). The preliminary clarification is used first to remove coarse components from the wastewater mechanically or by simple sedimentation. The coarse components which are separated off must be pumped off at regular time intervals. The activation tank contains microorganisms which perform the biological purification, the secondary clarification tank serves for separating off the microorganisms which have been introduced and their recirculation to the activation tank and/or to the preliminary clarification tank. This equipment can be supplemented, for example, by a compressor which feeds oxygen to the activation tank via a membrane tube aerator, and also a submersible motor pump for transporting the excess sludge.
In an activation tank, in the course of time, excess clarified sludge forms which must be regularly drawn off by suction. Alternatively, what are termed moving-bed processes are also known in which the amount of excess sludge is generally drastically reduced. In contrast to the classical activation tank, in moving-bed processes, free-floating plastic bodies are employed which can fill up the activation tank virtually completely. Microorganisms are found in this case both on the plastic bodies and also freely suspended in the water.
In recent years, as a further development of the known moving-bed methods, increasingly what are known as WSB® methods (fluidized bed-moving bed-biofilm methods) have been able to become established. Such methods are described, for example, in DE 10127554 and also in DE 196 23 592. In those methods also, plastic bodies serve as carriers which can be colonized by the microorganisms. The microorganisms in WSB® methods are, however, generally localized virtually completely on the carrier material. Whereas in fluidized-bed methods operations were originally carried out exclusively anaerobically (without aeration), in WSB® methods, as a result of the introduction of air, the plastic carriers colonized by microorganisms are optimally and uniformly distributed (or “fluidized”) in the activated zone and are situated in the moving suspension, which gave rise to the name Wirbel-Schwebe-Bett (WSB®) (fluidized bed-moving bed). Even in the case of greatly varying inflow, e.g., in the holiday period, the biological system in the activation tank always remains intact.
However, there is also the problem in the case of wastewater purification by a WSB® method that microorganisms are discharged into the environment, that is to say microbe-containing wastewater is introduced into the activated soil zone or into a receiving body of water.
To avoid harmful microorganisms from being introduced into the activated soil zone and/or to enable treated wastewater to be able to be reused as service water, the wastewater must therefore be additionally filtered.
For instance, DE 19807890 described a sewage treatment plant, the wastewater of which is filtered through submerged microfiltration membranes to pass it thereafter into a service water reservoir and reuse it. DE 20315451 describes a microfiltration device as a retrofitting set for a small sewage treatment plant which is connected downstream of the activation tank but upstream of the actual outlet of the sewage treatment plant.
In those cases, organic filter membranes are used which are arranged in a module form. However, organic filter membranes have the disadvantage that they can only be regenerated or chemically cleaned inadequately, so that all of these membranes must generally be renewed in relatively short time periods (<1 year). In addition, organic membranes have only a restricted mechanical stability, such that at relatively high liquid pressures, they can easily be damaged. The use of organic membranes becomes particularly serious in the above-mentioned moving-bed methods, in particular, in the WSB® method, since in these, as a result of the plastic carrier particles which can move freely in the activation tank, mechanical defects on the organic membrane can be caused as a result of which the low-stability organic filter membranes can be destroyed within a short time.
The use of filter membranes is in addition linked with the fundamental problem that during filtration, a covering layer is deposited on the outer surface of the membrane (what is termed “fouling”) which provides resistance to the material to be filtered. This leads to a drastic reduction in filter performance to total blockage and, therefore, to total loss of the filter membrane.
Detachment of this covering layer on the membrane requires regular cleaning. In this operation the permeate stream is reversed so that the previously filtered water is then pumped back through the filter membrane in the opposite direction (backwashing). As a result, the covering layer is at least in part detached, as a result of which the efficiency of the filter performance is increased again for a certain time. However, this procedure generally requires separate equipment. In addition, cleaning is at the cost of losing already filtered water which greatly reduces the efficiency of the overall system.
It could therefore be helpful to provide a simple and inexpensive solution for purifying wastewaters.